Visually suppress summaries & keep track of them so they may be removed when needed

When an actual group is swiped away, we keep the NotifEntry around so
that the app can cancel that group. This CL keeps track of those entries,
visually suppresses them, and removes them when all of the bubbles of that
group are also removed.

Test: manual
      1) have a group of bubbles with summary
      2) swipe away summary => observe that bubbles remain and
         that the summary remains hidden
      3) check dumpsys Bubble Controller => observe the summary suppressed
         check dumpsys Notification List => observe summary is in the list
      4) dismiss the bubble stack,
         check dumpsys Bubble Controller => summary key no longer there
         check dumpsys Notification List => summary is not in the list
Change-Id: Ib195c5cfa7099fbf8ec210024e11af4702337154